<script src="/vue.js"></script>

<body>
    <div id="app">
        <input type="text" v-model="cityName"/>
    </div>
</body>
    <script>
        var vue=new Vue({
            el:"#app",
            data:{
                cityName:"天津"
            },
            watch:{
                cityName(newName,oldName){
                    console.log(newName,oldName);
                }
            }
        });
    </script>